The invention relates to a method for operating a plant for producing cement clinker from raw meal having, viewed in the material flow direction, at least one calcinator for deacidifying the raw meal, and at least one rotary kiln for sintering the deacidified raw meal to form cement clinker, wherein the deacidified raw meal, after passing the calcinator, flows via a cyclone preheating stage into the rotary kiln.
To produce cement clinker, a mixture of calcareous rock and silicate-containing rock is ground and subjected to a heat treatment, during which the limestone is formally freed of carbon dioxide (CO2) and converted into burnt lime (CaO). In a further stage, the raw meal, which is deacidified by being freed from CO2, and which consists of the deacidified calcareous rock and the silicate-containing rock, which is still unchanged up to this point, is sintered in the heat to form various calcium silicate phases.
The deacidification and also the sintering of raw meal are endothermic processes, which require thermal energy for their reaction. This thermal energy can be obtained from high-quality fuels. In addition to the classical primary fuels, for example, coal, alternative fuels, which are frequently obtained from municipal or industrial wastes, are increasingly used as energy carriers in cement plants for reasons of cost.
Both the high-quality fuels and also the alternative fuels result in the formation of nitrogen oxides (NOx) during the combustion to supply the processing heat. The type of the thermal treatment mentioned at the outset makes it necessary for the sintering to be performed in a rotary kiln, wherein very high temperatures of at least 1450° C. must prevail in the rotary kiln for successful sintering of the calcium silicate phases. To generate these high temperatures in the rotary kiln, flame temperatures which reach up to 1800° C. are indicated. At the high temperature, both nitrogen occurring in the fuel, usually in the form of amines, and also air nitrogen occurring in the combustion air, are combusted to form nitrogen oxides (NOx). If no measures are taken to avoid or reduce the occurring nitrogen oxides, the nitrogen oxides escape with the exhaust air of the rotary kiln into the free atmosphere, where they are converted by hydrolysis with the ambient humidity to form nitric acid (HNO3), nitrous acid (HNO2), and other acidically reactive nitrogen oxide hydrates. The nitrogen oxides (NOx) which acidically react with ambient humidity are the primary cause of undesired acid rain, which decreases the natural pH value of forest soil and weakens its resistance to illnesses. Various measures are known for reducing the emission of nitrogen oxides (NOx) from plants for producing cement.
A plant for producing cement, which has, in addition to the calcinator, a combustion chamber, which is fed with tertiary air as combustion air, is disclosed in German published application DE 10 2005 057 346 A1. The reduction properties of the exhaust gases produced in the combustion chamber can be set very precisely in the plant disclosed therein, to reduce the nitrogen oxides originating from the rotary kiln in the calcinator in the calcinator section. The method disclosed therein has the disadvantage that the deacidification of the raw meal in the calcinator, on the one hand, and the reduction of nitrogen oxides also in the calcinator, on the other hand, find their desired chemical equilibrium at different temperatures. The deacidification and the reduction of nitrogen oxides mutually interfere with one another.
A skein calcinator is disclosed in German published application P 35 38 707 A1, which has two reaction sections in the form of gas skeins, which rise adjacent to one another within the calcinator, of which a first gas skein flows from the rotary kiln into the calcinator and a gas skein adjacent thereto flows from a second infeed of the tertiary air into the calcinator. The gas skeins have different reduction potentials and the gas skein from the tertiary air is to reduce the nitrogen oxides in the gas skein from the rotary kiln. According to the teaching described therein, a relatively short time is available for burning out the nitrogen oxides, so that the requirement for the control of the flow conditions within the calcinator is very high. Since the flow conditions in the calcinator can vary in the event of slight changes of the operating parameters, this operating mode is not always in the optimal state.
A principle similar thereto is disclosed in German published application DE 199 03 954 A1, wherein a reactor for setting the redox potential of the gases to be generated is present in the tertiary air section. This operating mode also places a high demand on the control of the flow conditions in the calcinator. Since the flow conditions in the calcinator can also vary in the event of slight changes of the operating parameters in this plant, this operating mode is also not always in the optimal state.
A calcinator having top air supply is disclosed in German published application DE 199 62 536 A1. In this calcinator, a staged combustion is carried out, during which nitrogen oxide (NOx) is reduced by carbon monoxide (CO) formed inside the combustion section. To improve the nitrogen oxide reduction while simultaneously avoiding carbon monoxide in the exhaust gas of the plant, it is proposed that a reducing agent and/or a catalyst be injected in dependence on the measured exhaust gas parameters in the calcinator section.
A rotary kiln is disclosed in EP 1 334 954 B1, which, in addition to the rotary kiln, which is already provided for the clinker firing, of a plant for producing cement, carbonizes coarse, lumpy fuel. The pyrolysis gases and the hot air generated by the carbonization are blown into the calcinator, to thus assist the heat supply for the deacidification carried out in the calcinator. With correspondingly little air supply, the carbonization gases have a reductive effect and can be used for reducing the nitrogen oxides in the calcinator, which originate therefrom. However, the operation of a second rotary kiln is linked to a high structural expenditure and requires continuous monitoring of the mechanism, which is always hot in operation.
A plant for producing cement clinker from raw meal with utilization of waste materials having a high caloric value is known from DE 35 33 775 C1, in which waste materials used as a secondary fuel are thermally treated after their drying in a carbonization furnace, which is operated with rotary kiln exhaust gas and a partial stream of the tertiary air, for pyrolysis, but at least for partial combustion. The pyrolysis gas of this combustion is introduced into the calcinator and the solid pyrolysis residues are at least partially introduced into the rotary kiln after their preparation and homogenization. The carbonization furnace can also be implemented as a rotary kiln in this case.
In a plant for producing cement clinker with utilization of waste materials having a high caloric value, carbonizing or combusting the waste materials in a separate rotary kiln and using the carbonization gas/exhaust gas during the thermal raw meal treatment is also known from DE-A-33 20 670, DE-A-34 11 144, and DE-A-35 20 447.
All previously known plants and the methods embodied therein are directed to a chemical reduction of the rotary kiln exhaust gases in the calcinator in the presence of the raw meal to be deacidified, however, with the disadvantages mentioned at the outset of the short dwell time, the interfering deacidification reaction, and the high demands on the control of the flow conditions. The temperature windows, which are required for the different reactions occurring simultaneously in the calcinator and nearly at the same location, are difficult to produce. The deacidification of the calcium carbonate (CaCO3) contained in the raw meal to form burnt lime (CaO) occurs at lower temperatures than the ideal temperature window for the denitrification during the staged combustion. Finally, the temperature window for the generation of carbon monoxide (CO) is in turn different from that for denitrification.
It is known that the denitrification, i.e., the reduction of nitrogen oxides (NOx) to form elementary nitrogen (N2) and oxygen (O2), is dependent on the further gas composition in the exhaust gas to be denitrified. The denitrification decreases rapidly with rising CO2 concentration. In contrast, the denitrification increases with a rising concentration of carbon monoxide (CO) in the exhaust gas to be denitrified. The concentration of (CO2) and (CO) are in the Boudouard equilibrium in a hot exhaust gas. Better control of the exhaust gas composition of the rotary kiln, which has a high nitrogen oxide component, so-called air nitrogen, due to the high combustion temperature, would be desirable. The exhaust gases of the rotary kiln are not immediately discarded, but rather used for the deacidification of the raw meal.